1. Technical Field of the Invention
The present invention relates to a piston ring used in internal combustion engines.
2. Description of the Related Art
In recent years, engines must meet increased demands for higher output, alternative fuels and longer product service life and must also comply with even stricter exhaust gas emission regulations. All these circumstances lead to a reduction in the oil film thickness between the piston ring and the cylinder wall, leading to a higher direct contact of the mating surfaces. The piston ring must therefore work in a more severe operating environment, which requires higher scuffing and wear resistance.
Considering the current arrangement of piston rings, the top ring is the most stressed concerning high pressure and high temperatures, due to its closeness with the combustion chamber; also it is the ring that works with the thinnest oil film. Therefore, top rings are usually nitrided.
Gas Nitrided Steel (GNS) is a common technology used for top rings in Otto engines. Gas nitriding is a case-hardening process whereby nitrogen is introduced into the surface of solid ferrous alloys by holding the metal at suitable temperature in contact with a nitrogen containing gas, usually ammonia. The process of nitriding steels produces less distortion or deformation than either carburizing processes or conventional hardening processes. Main reasons for nitriding are to obtain piston rings with high surface hardness, to increase wear resistance and scuffing resistance properties and to improve fatigue life.
However, in more severe cases, even the hard nitrided layer of the conventional art is not resistant enough to avoid scuffing. This may occur especially on flex-fueled vehicles (FFV) engines, where the combustion strategy has to address different fuels. In some conditions, richer fuel mixtures can cause cylinder fuel washing, leading to lack of oil film, and consequently to a strong contact between the mating parts which is prone to scuffing occurrence.
In an attempt to minimize the above-described scenario, U.S. Pat. No. 6,726,216 discloses a piston ring comprising a region of increased nitride content, whereby the piston ring is oxynitride-hardened, comprising an oxide layer, which is formed over the nitrided layer. During operation, said oxide layer is in sliding contact with the cylinder wall and it reduces the friction coefficient, consequently improving the sliding functions during the run-in of the engine. Another advantage of the oxide layer is the increase in the material resistance against corrosive media. Despite of such improvements, there are still limitations concerning scuffing occurrence on oxygen-containing layers because a porous rigid area is developed in the outer region of the composite layer surface, negatively influencing the operational behavior of the components stressed by sliding friction.
Therefore, nitrided rings can become prone to scuffing when exposed to these conditions. Surface treatments and/or coatings have been applied to nitrided rings in order to overcome these problems, improving nitrided layer performance.
U.S. Pat. No. 6,279,913 discloses a method for applying amorphous carbon, i.e., a diamond-like carbon thin film, which is formed directly on a gas nitrided layer, by means of ion plating or reactive sputtering process. Nevertheless, when the irregularities on the surface to be covered are too fine, the diamond-like carbon film has a surface structure with a smooth layer shape and lack of adhesion is prone to occur. Further, when the irregularities on the surface to be covered are too large, the surface roughness of the diamond-like carbon film becomes higher so that the sliding characteristics degrade and peeling or film collapse is prone to occur, besides the burden of a high cost production.
Moreover, U.S. Pat. No. 5,316,321 describes a ceramic coating of chromium/titanium nitride deposited by a physical vapor deposition process over a nitrided layer. These ceramic coatings complement the hard gas nitrided layer by providing advantages not possessed by only the gas nitrided layer, such as higher scuff resistance and extremely high hardness. Yet, the desirable advantages offered by metallic nitride face coatings result in unwanted side effects. These coatings are particularly prone to chipping and may not achieve suitable coating thickness in case they lack sufficiently strong bonds with an underlying substrate. Since the TiN/CrN films are brittle due to their high hardness, excessive repetitive stress on the sliding surfaces during operation may generate cracks, causing peeling and loss of localized pieces of the coating.
In order to overcome the problems of the above-mentioned surface technologies, it is necessary to provide a solution with a superior scuffing and wear resistance and a competitive cost.
Considering other treatment techniques for protecting sliding surfaces, applied with regard to components other than piston rings, the so-called sulphonitriding treatment is disclosed in U.S. Pat. No. 6,804,944. More specifically, a spinning machine traveler comprising a sulphide layer over a nitrogen compound layer is disclosed, both layers being formed over a base material consisting of hard steel. After the sulphide layer has been worn away, due to normal working conditions, the nitrogen compound layer takes place, making it possible to perform spinning operation in a more stable manner, elongating the service life of the traveler. However, for piston ring applications, premature wear of the outmost surface leads to nitrided layer exposure, going back to the original problem of scuffing generation.
In addition, U.S. Pat. No. 5,985,428 discloses a method to treat working die steel members, forming a surface layer by a sulphonitriding process, consisting of an oxygen-containing mixed layer, comprising iron sulphide and iron nitride particles. Sulphur and nitrogen contents on the mixed layer satisfy the formula 0.5≦S/N≦10.
Knowing that S is directly related to the formation of sulphides and N to the formation of nitrides, derived from the S/N relation, it can be inferred that in most of the cases, there is a high proportion of sulphides compared to nitrides. This high sulphide proportion promotes a layer with low wear resistance, due to low load carrying capacity of sulphides under sliding conditions. As explained before, one can expect poor scuffing resistance from oxygen-containing layers, as the one cited here containing from 1 to 15 wt % of oxygen.
Furthermore, U.S. Pat. No. 5,187,017 discloses a sliding member comprising Fe as a matrix and comprising a FeS2 (ferric sulphide) layer on the topmost surface of the sliding member. According to this patent, FeS2 is capable of retaining more lubricant as can be held by FeS, due to its inherent porosity. However, FeS2 phase, for being a soft and continuous layer, presents high wear rate in high load carrying capacity applications. Also, FeS2 layers easily peel off from an iron nitride layer.
The last three references mentioned are not related to the use of the sulphonitriding technology on internal combustion engine parts, particularly on piston rings, and none of them present both resistance to scuffing and wear.
Piston rings are designed to seal the combustion chamber from the crankcase (blow-by) and transfer heat from the piston to the cylinder. The rings also assist in controlling engine oil consumption, which has a strong influence on exhaust gas emissions. The effects of contact temperature and surface roughness on friction coefficient have an important role on the start up of the engine and this environment is prone to scuffing occurrence.
The selected surface treatment must be able to provide the above performance characteristics over the run-in of the engine and must be compatible with the cylinder wall and show optimal wear, friction, and scuff-proof behaviors as established by current engine demands.
The running face of a piston ring requires a material that reduces contact friction as much as possible and still allows the ring to perform its sealing function against the cylinder wall. Even though the ring's running face is usually sliding on a very thin layer of oil, in some points of the track there is lack of oil where the friction coefficient becomes higher. Minimizing such friction losses is a challenge, mainly at the run-in of the engine where the combined roughness of the mating parts (running face of the piston ring and the cylinder wall) is much higher than after break-in.